Sensors are devices that respond to a stimulus and produce a signal indicative of the stimulus's magnitude, relative position, or other characteristic attributable to the stimulus. The stimulus may be any physical quantity or parameter which can affect a sensor. An array of sensors is a collection of individual sensors positioned at discrete locations to form a relative field of perception.
One type of sensor array is a capacitive sensor array. A capacitive sensor is used to detect the presence and/or absence of a conductive object, although direct contact between the conductive object and sensing element is not necessarily required. Capacitive sensors are typically constructed of a conductive pad, the surrounding ground, and its connection to a controller. In most applications, the conductive pad is a large copper footprint and the surrounding ground is a poured fill. A native (parasitic) capacitance exists between these two objects. When a third conductive object, a stimulus—such as a human finger—is brought into proximity with the sensor, the capacitance of the system is increased by the capacitance of the stimulus. Capacitive sensors are generally resistant to environmental factors, such as water, temperature and humidity, and may be used with a variety of overlay materials and thicknesses.
A capacitive sensor array typically employs a number of discrete capacitive sensors distributed over a region of the array which may be arranged in a pattern forming a grid. A grid of sensors may comprise a plurality of capacitive sensors which may be individually addressable, addressable in subsections of the grid, or in their entirety. Addressing specific sensors may be accomplished using multiplexers coupled to the sensor array according to data or select signals on multiplexer select lines to determine the individual sensors to be “driven” or “sampled.”
A sensor is driven by exciting the sensor or energizing the sensor so as to produce a measurement of the stimulus at the sensor. By sampling a sensor, an output signal from the sensor is read to detect the sensor response to the stimulus. Thus, it is possible to selectively measure a signal representative of the sensor capacitance from a given capacitive sensor element located at a particular column and row of the capacitive array. Multiplexers may be used to determine the particular row and column from which a measurement is desired. These grids may be adapted to form interfaces of various sizes and shapes. For example, rectangular touch pads are common in PDAs and mobile handsets. Other embodiments include linear and radial interfaces.
Applications in which such sensor arrays are useful include touch pads and distributed sensors that provide an indication of the location and magnitude of a force or a pressure applied to a region of interest. These applications may be used in user interfaces of computing devices, such as notebook computers, personal data assistants (PDAs), and mobile handsets. Other applications in which sensor arrays have been incorporated include kitchen appliances, exercise equipment and other consumer electronics.
As consumer electronic devices continue to reduce in size, so too, do their user interfaces. A smaller capacitive sensor user interface typically means smaller individual capacitive sensors within the user interface. Generally, shrinking the size of a capacitive sensor adversely affects its sensitivity, resulting in a detrimental effect on the user experience (such as slower response time, reduced accuracy). Decreased sensitivity due to shrinking sensor size can be partially compensated by increasing the sampling time of a particular capacitive sensor. However, increasing the sampling time for each capacitive sensor within an array of capacitive sensors reduces the response time of the user interface while simultaneously increasing the rate of power consumption. For mobile devices which have a limited power source (e.g. battery-powered devices), this can contribute to a reduced user experience.